1. Field of the Invention
The present invention relates to a laser beam machining apparatus for forming a tearable portion in a workpiece such as automotive instrument panel by irradiating a laser beam and a laser beam machining method therefor.
2. Description of the Related Art
Air bags for passenger seats of automobiles are generally placed on the inside of instrument panels. Moreover, an opening is formed in the instrument panel in a position opposite to the air bag and a cover plate is mounted over the opening; or otherwise there is formed a groove for use in cleaving a predetermined opening portion in the instrument panel though no opening is provided therein. In this case, the air bag is normally covered with the cover plate or the opening portion and when the air bag is activated, the cover plate is broken or the opening portion is cloven out of the groove so that the air bag is inflated outside the instrument panel.
With the above structure of covering the air bag, however, the problem of spoiling the external appearance arises as the outward form of the cover plate or the groove of the opening portion is exposed on the front surface of the instrument panel. A further problem is that the reflection of the outward form of the cover plate or the grooves of the opening portion on the front window also puts a restraint on driving activity.
In order to deal with the foregoing problems, there has been proposed a method of forming a tearable portion by drilling blind holes at predetermined intervals in the rear side of an instrument panel to form a line of perforations. In this method, however, it is needed to form the tearable portion having a predetermined value of breaking strength over the whole periphery while a predetermined remain thickness is secured between the bottom face of each perforation and the front surface side of the panel.
Nevertheless, because instrument panels are curved and of uneven thickness when they are produced by molding, it has been difficult to drill blind holes in the instrument panel so that the remain thickness is secured uniformly.
In order to deal with the problem above, there have heretofore been proposed laser beam machining methods as disclosed in, for example, Japanese Patent Laid-Open No. 85966/1998 (the first prior art) and Japanese Patent Laid-Open No. 2001-38479 (the second prior art).
According to the first prior art, when a tearable portion as a blind hole is drilled by the irradiation of a laser beam to a workpiece, the laser beam transmitted through the aperture is detected whereby to stop the drilling of the aperture when the power transmitted therethrough reaches a predetermined value. According to the second prior art, further, a laser beam is synthesized with a detection beam beforehand and the detection beam transmitted through an aperture is detected when the aperture is drilled by irradiating the laser beam to a workpiece whereby to stop the drilling of the aperture when the power transmitted therethrough reaches a predetermined value.
As set forth above, according to the conventional laser beam machining methods, the depth of apertures to be drilled is controlled so that the remain thickness is secured uniformly by detecting a small quantity of laser beam or detection beam transmitted through the cut part (aperture) of the workpiece. Therefore, a sensor for detecting the laser beam or the detection beam is needed to be provided on a supporting member such as a jig for supporting a workpiece to be subjected to machining and in a position corresponding to a laser nozzle in such a manner that the sensor is movable integrally with the workpiece on the opposite side of the workpiece, and this makes the detection device not only complicated in construction but also expensive. The problem in this case is that the necessity of adjusting the detection level before the workpiece is subjected to machining makes it troublesome to detect slight variation in the thickness of the workpiece. According to the first prior art, moreover, there is the possibility that as the laser beam is transmitted through the aperture of the workpiece, a small hole or a scar is bored or left in the base of the aperture in the form of a tearable portion.
Further, in order to make constant the remain thickness, the output power of the laser beam has to be stabilized. For the stabilization of the laser beam, various factors or conditions such as the voltage supplied to the oscillator of a laser beam machining apparatus, the temperature of cooling water in an optical system and the ambient temperature of the machining apparatus also have to be definite. Actually, these factors and conditions are hardly kept constant and with the change of the optical system with time, moreover, the actual output power of the laser beam tends to vary in most cases.
In case where laser beam machining is carried out while the output power of the laser beam remains variable, the remain thickness of a workpiece also varies and this may result in impeding development of cleavage when the air bag is inflated, thus making small holes or leaving scars in the workpiece because a laser beam is allowed to pass through the workpiece.
In order to compensate for the variable output power of the laser beam, there has been proposed the art described in, for example, Japanese Patent Laid-Open No. 278687/1988 (the third prior art). In this prior art, the output power of a pulsed laser beam following a reference pulse oscillated at a predetermined fundamental frequency as shown in FIG. 12 is detected pulse-to-pulse and the output power is integrated by an integration circuit. Then the integrated value is compared with a reference value whereby to decide the error therebetween.
However, though discharge time is required for the integration circuit when the output of the pulsed laser beam is integrated, the discharge time is difficult to secure because the output OFF time of the pulsed laser beam oscillated at a high speed of 5 KHz, for example, is too short.
Therefore, it is conceivable to shorten the time constant of the integration circuit but as is obvious from an integral value 30 of FIG. 12, the oscillation noise of the pulsed laser beam is detected. In case where the time constant is set longer, on the other hand, the discharge time of the integration circuit would not be secured as stated above. Consequently, a value widely different from the actual output power value is monitored and the laser output power cannot be detected accurately according to the prior art method. When the prior art method is applied to forming a tearable portion in the instrument panel, it becomes impossible to control the laser output power and this results in difficulty in making constant the remain thickness of the panel.
A first object of the invention made with a special attention directed to the foregoing problems existing in the prior art is to provide a laser beam machining apparatus for ensuring that slight variation in the thickness of a workpiece is made detectable by a detection device which is simple in construction and producible less costly and for accurately drilling perforations in a panel while the remain portion of the panel base secures the predetermined thickness, and a laser beam machining method therefor.
Further, a second object of the invention is also to provide a laser beam machining apparatus for making constant the remain thickness of a panel base by allowing the value of a laser beam by pulse oscillation to be accurately securable and a laser beam machining method therefor.
In order to accomplish the first object, according to a first aspect, there is provided a laser beam machining apparatus for forming a blind hole in a workpiece by the irradiation of a laser beam from a nozzle to a rear surface of the workpiece while moving a machining head having the nozzle and the workpiece relatively, comprising:
a workpiece support member made of conductive material for supporting and fixing the workpiece;
a distance detection unit provided in the machining head and detecting the electrostatic capacity of a distance with a surface of the workpiece support member as a reference; and
a control unit controlling the laser beam in response to the electrostatic capacity which varies along with the thickness of the workpiece disposed between the workpiece support member and the distance detection unit.
Therefore, according to the first aspect of the invention, a detection device simple in construction is producible less costly contrary to the prior art in which the laser beam or detection beam transmitted through the blind hole of the workpiece is detected. Moreover, it is not only ensured to make detectable slight variation in the thickness of the workpiece but also possible to accurately form the blind hole while the predetermined remain thickness of the bottom face of the panel base is secured.
According to a second aspect of the invention, in the laser beam machining apparatus according to the first aspect, the laser output power by pulse oscillation is controlled in response to variation in the electrostatic capacity while the nozzle and the workpiece are moved relatively with a predetermined distance between the workpiece support member and the distance detection unit being made constant.
Therefore, according to the second aspect, the depth of the drilling hole can be adjusted easily and accurately by setting the number of oscillated pulses of the laser beam in response to variation in the electrostatic capacity. Further, many perforations are formable at the predetermined intervals while the predetermined remain thickness of the panel base is secured, which is fit for use in providing an opening for an air bag in an instrument panel for covering the air bag.
According to a third aspect of the invention, in the laser beam machining apparatus according to the first aspect, the distance detection unit is provided to the nozzle.
Therefore, according to the invention described in the third aspect, no special mount need not be provided for the distance detection unit, which is made simple in construction, whereby the accessibility of the nozzle to the workpiece is improved.
According to a fourth aspect of the invention, there is provided a laser beam machining method for forming a blind hole in a workpiece by irradiating a laser beam from a nozzle to a rear surface of the workpiece while moving a machining head having the nozzle and the workpiece relatively, comprising the steps of:
fixing the workpiece to a workpiece support member made of conductive material;
detecting the electrostatic capacity of a distance with a surface of the workpiece support member as a reference; and
controlling the laser output power by pulse oscillation in response to the electrostatic capacity which varies along with the thickness of the workpiece disposed on the workpiece support member, so that the bottom portion of the hole has a predetermined thickness.
Therefore, the same function as what is described in the first aspect can be retained according to the invention described in the fourth aspect.
According to the fifth aspect of the invention, the laser beam machining method according to the fourth aspect, further comprises the step of:
performing a peripheral groove by oscillating pulses intermittently at predetermined intervals when the machining head and the workpiece are moved relatively.
Therefore, the same function as what is described in the second aspect can be retained according to the invention described in the fifth aspect.
According to the sixth aspect of the invention, the laser beam machining method according to the fourth aspect, further comprises the step of:
adjusting the drilling depth by controlling the laser output power in response to the number of pulses of the laser output by pulse oscillation.
Further, in order to accomplished the second object, according to a seventh aspect of the invention, there is provided a laser beam machining apparatus of irradiating a laser beam by pulse oscillation from a nozzle to a rear surface of a workpiece while moving the nozzle and the workpiece relatively, so that blind holes are lined up in the workpiece with remain portions in a front surface side of the workpiece, comprising:
a thickness detection unit detecting the thickness of the workpiece,
an output power control unit controlling the output power of the laser beam in response to the result detected by the thickness detection unit so that the irradiation of the large power laser beam is switched to the irradiation of a small power laser beam after the irradiation of the large power laser beam at each perforation;
a laser beam detection unit detecting the actual output power of the laser beam when the large power laser beam is irradiated,
an adder adding the output power detected by the laser beam detection unit; and
an adjusting unit adjusting the output power of the small power laser beam irradiated after the irradiation of the large power laser beam to conform to a command value in response to the result added by the adder.
Therefore, according to the seventh aspect of the invention, machining blind holes with the remain portion can be formed in the workpiece for machining the tearable portion by the irradiation of the laser beam by pulse oscillation. Then the machining blind holes are continuously lined up in the form of the tearable portion along the surface of the workpiece so as to form a groove-like tearable portion. In this case, the small power laser beam by pulse oscillation is used to machine the blind holes after the large power laser beam thereby is used to machine the blind holes and the number of pulses of the small power laser beam is so adjusted as to conform to the command value in accordance with the laser beam output by the large power laser beam. Consequently, even though an error in the depth of the blind hole is made because of the large power pulsed laser beam, the error can be rectified at the time of drilling a hole by means of the small power laser beam, so that the remain thickness of the bottom face of the blind hole is uniformized. As the small power laser beam is used to drill holes after the large power laser beam is used to drill holes, the opening side of each hole is widely formed. Hence, inactive gas and swarfs of the workpiece are smoothly discharged, whereby the drilling operation can be performed precisely and efficiently. In making a decision on the laser beam output power, further, the actual output power can be accurately calculated only by adding the pulse-to-pulse power of the laser beam by pulse oscillation. On the basis of the accurate power value, the drilling depth, that is, the accurate remain thickness of the panel base can be secured. Moreover, the depth of the blind holes is made adjustable by adjusting the laser beam output power by pulse oscillation in response to the thickness of the workpiece and even though the thickness of the workpiece varies, the uniform thickness of the remain portion of the panel base can be secured.
According to an eighth aspect of the invention, in the laser beam machining apparatus according to the seventh aspect, the thickness detection unit includes an electrostatic capacity sensor provided at the front end of the nozzle.
Therefore, in comparison with a case where the laser beam transmitted through the workpiece is detected, it is ensured that the thickness of the workpiece is detectable without cause through-holes to appear in the workpiece. Moreover, the sensor is not needed to be installed on both sides of the workpiece and this makes the laser beam machining apparatus simple in construction.
According to a ninth aspect of the invention, in the laser beam machining apparatus according to the seventh aspect, the irradiation power of the small power laser beam at each drilling is set in a range between 70% or lower and 20% or higher of the irradiation power of the large power laser beam.
Therefore, in the invention described in the ninth aspect, since the small-output irradiation power is sufficiently lower than the large-output irradiation power, the depth of the blind hole is made adjustable by the small power laser beam.
According to a tenth aspect of the invention, there is provided a laser beam machining method of irradiating a laser beam by pulse oscillation from a nozzle to a rear surface of a workpiece while moving the nozzle and the workpiece relatively, so that blind holes are lined up in the workpiece with remain portions in a front surface side of the workpiece, comprising the steps of:
detecting the thickness of the workpiece;
switching the irradiation of a large power laser beam to the irradiation of a small power laser beam after the irradiation of the large power laser beam at each perforation in response to the detected result of the thickness of the workpiece;
detecting the actual output power of the laser beam when the large power laser beam is irradiated;
adding the detected output power; and
adjusting the output power of the small power laser beam irradiated after the irradiation of the large power laser beam to conform to a command value in response to the added result.
Therefore, the same effect as what is described in the seventh aspect is achievable.
According to an eleventh aspect of the invention, in the laser beam machining method according to the tenth aspect, the output power of the large power laser beam is corrected so that the output power thereof conforms to the command value in response to the added result.
Therefore, as the large power laser beam causing the drilling depth to be great is an object for correction, the depth of the holes can effectively be corrected.
According to a twelfth aspect of the invention, in the laser beam machining method according to the tenth aspect, the irradiation power of the small power leaser beam at each perforation is set in a range between 70% or lower and 20% or higher of the irradiation power of the large power laser beam.
Therefore, the same effect as what is described in the ninth aspect is achievable according to the invention described in the twelfth aspect.